Transmission of digital data messages in digital telephony

ABSTRACT

In a communications between a base station and subscriber unit in time slots within fixed length time frames, different types of messages require different numbers of timeslots per frame to maintain transmission rate. At least a portion of the message is detected to determine the type of message, and if appropriate, additional time slot(s) are assigned.

The invention relates to digital data transmission in time slots withinfixed length time frames, in particular, time division multiplexing/timedivision multiple access (TDM/TDMA) digital telephony.

Conventional TDM/TDMA systems, such as GSM ("Global System for Mobilecommunications") systems are not suited to the transmission ofnon-speech data and facsimile. As discussed in Cellular Radio Systems, DM Balston & R C V Macario (Editors), Artech House Inc. 1993, Chapter 14,Pages 315 to 323, GSM equipment performs erratically in transmittingGroup 3 facsimile signals, and cannot handle Group 4 facsimile. A keyproblem is the limited bandwidth which is available. Each data ortelefax transmission is sent at a conventional rate in a designated slotin successive TDMA frames. Also, ISDN (Integrated Services DigitalNetwork) transmissions require increased bandwidth.

The present invention is defined in the claims, to which referenceshould now be made.

The present invention preferably provides a method of transmitting adigital data message between a sending unit and a receiving unit using aTDMA data transmission scheme, a slot or slots in successive TDMA framesbeing assigned for transmission of the message, in which at least aportion of the message is detected to determine a parameter of themessage, at least one further slot is selectively assigned fortransmission of the message dependent on said parameter. The message isallocated to the assigned slots for transmission then reconstituted atthe receiving unit. The first slot or slots and the further slot orslots are preferably included in each frame.

The number of slots which are used to carry a message can be increased,or decreased, without terminating the call connection between thesending unit and the receiving unit. The effect of adding an additionalslot or slots is to provide more bandwidth within which to transmit themessage. As the assignment of slots is varied depending on need, it isnot necessary to have full knowledge, when a call is first established,of what bandwidth might be required.

Preferably, the sending unit and receiving unit are a base station andsubscriber unit. The assignment of slots is preferably undertaken by thebase station. When a call is first established, the system does not knowwhether the message is speech on the one hand, or non-speech voice-banddata on the other hand, such as from a modem or facsimile machine, or anISDN message. Facsimile messages are preferably carried in two slots perframe unless a second slot is unavailable due to the large amount ofcommunication traffic on the system at that time. Using two slots perframe effectively doubles the data transmission rate. ISDN messages arepreferably carried on two to five slots per frame, with thecorresponding multiplication of transmission rate.

The detection process preferably occurs at the base station to determinethe parameter which indicates whether a faster transmission rate isdesirable. If so, a decision is made to assign a further slot or slots.

A detection process occurs at the base station to determine whether themessage being transmitted in a first slot is speech or voice-band data.A detector at the base station can perform this function, in particular,by detecting whether the message includes standard modem answertone orfacsimile handshake or ISDN call set-up signalling or any othercharacteristic expected if the call includes voice-band data. If thischaracteristic is detected, a decision to assign an additional slot orslots is made.

The present invention also preferably provides communication means forcommunicating a digital message using a TDMA data transmission scheme,the communication means including a sending unit and a receiving unit,at least one of said units including first slot assignment meansoperative to assign a slot in successive TDMA frames, and second slotassignment means operative to selectively assign a second or subsequentslot, the sending unit including means operative to divide the messagebetween the assigned slots, and the receiving unit including meansoperative to reconstitute the message.

The present invention in another aspect preferably provides a basestation comprising transmission means for transmission of TDMA digitaldata signals and reception means for receiving TDMA digital datasignals, the base station including means to assign a first slot in aframe for communication with a subscriber unit, means to detect at leasta portion of the transmitted signal to determine a parameter of thetransmitted signal and means to selectively assign a second orsubsequent slot for communication with the subscriber unit dependent onthe sampled parameter.

A preferred embodiment of the invention will now be described, by way ofexample, with reference to the drawings in which:

FIG. 1 is a schematic diagram illustrating the system including a basestation (BTE--Base Terminating Equipment) and subscriber unit(NTE--Network Terminating Equipment);

FIG. 2 is a diagram illustrating frame structure and timing for a duplexlink;

FIG. 3 is a diagram illustrating one example of how a digital message isassigned to multiple slots in a TDMA frame;

FIG. 4 is a diagram illustrating communications between a base station(BTE--Base Terminating Equipment) and subscriber unit (NTE--NetworkTerminating Equipment) in assigning a second slot;

FIG. 5 is a diagram illustrating communications between a base station(BTE) and subscriber unit (NTE) in re-assigning a second slot which isno longer required.

The Basic System

As shown in FIG. 1, the preferred system is part of a telephone systemin which the local wired loop from exchange to subscriber has beenreplaced by a full duplex radio link between a fixed base station (BTE)and fixed subscriber unit (NTE). The preferred system includes theduplex radio link (Air Interface), and transmitters and receivers forimplementing the necessary protocol. There are similarities between thepreferred system and digital cellular mobile telephone systems such asGSM which are known in the art. This system uses a protocol based on alayered model, in particular the following layers: PHY (Physical), MAC(Medium Access Control), DLC (DataLink Control), NWK (Network).

One difference compared with GSM is that, in the preferred system,subscriber units are at fixed locations and there is no need forhand-off arrangements or other features relating to mobility. Thismeans, for example, in the preferred system directional antennae andmains electricity can be used.

Each base station in the preferred system provides six duplex radiolinks at twelve frequencies chosen from the overall frequencyallocation, so as to minimize interference between base stations nearby.The frame structure and timing for a duplex link is illustrated in FIG.2. Each duplex radio link comprises an up-link from a subscriber unit toa base station and, at a frequency offset, a down-link from the basestation to the subscriber unit. The down-links are TDM, and the up-linksare TDMA. Modulation for all links is π/4--DQPSK, and the basic framestructure for all links is ten slots per frame of 2560 bits, i.e. 256bits per slot. The bit rate is 512 kbps. Down-links are continuouslytransmitted and incorporate a broadcast channel for essential systeminformation. When there is no user information to be transmitted, thedown-link transmissions continue to use the basic frame and slotstructure and contain a suitable fill pattern.

For both up-link and down-link transmissions, there are two types ofslot: normal slots which are used after call set-up, and pilot slotsused during call set-up.

Each down-link normal slot comprises 24 bits of synchronisationinformation followed by 24 bits designated S-field which includes an 8bit header followed by 160 bits designated D-field. This is followed by24 bits of Forward Error Correction and an 8 bit tail, followed by 12bits of the broadcast channel. The broadcast channel consists ofsegments in each of the slots of a frame which together form thedown-link common signalling channel which is transmitted by the basestation, and contains control messages containing link information suchas slot lists, multi-frame and super-frame information, connectionlessmessages, and other information basic to the operation of the system.

During the call set-up, each down-link pilot slot contains frequencycorrection data and a training sequence for receiver initialisation,with only a short S-field and no D-field information.

Up-link slots basically contain two different types of data packet. Thefirst type of packet, called a pilot packet, is used before a connectionis set up, for example, for an ALOHA call request and to allow adaptivetime alignment. The other type of data packet, called a normal packet,is used when a call has been established and is a larger data packet,due to the use of adaptive time alignment.

Each up-link normal packet contains a data packet of 244 bits which ispreceded and followed by a ramp of 4 bits duration. The ramps and theremaining bits left of the 256 bit slot provide a guard gap againstinterference from neighbouring slots due to timing errors. Eachsubscriber unit adjusts the timing of its slot transmissions tocompensate for the time it takes signals to reach the base station. Eachup-link normal data packet comprises 24 bits of synchronisation datafollowed by an S-field and D-field of the same number of bits as in eachdown-link normal slot.

Each up-link pilot slot contains a pilot data packet which is 192 bitslong preceded and followed by 4 bit ramps defining an extended guard gapof 60 bits. This larger guard gap is necessary because there is notiming information available and without it the propagation delays wouldcause neighbouring slots to interfere. The pilot packet comprises 64bits of sync followed by 104 bits of S-field which starts with an 8 bitheader and finishes with a 16 bit Cyclic Redundancy Check, 2 reservedbits, 14 FEC 9 bits, and 8 tail bits. There is no D-field.

The S-fields in the above mentioned data packets can be used for twotypes of signalling. The first type is MAC signalling (MS) and is usedfor signalling between the MAC layers of the base station and the MAClayer of a subscriber unit whereby timing is important. The second typeis called associated signalling, which can be slow or fast and is usedfor signalling between the base station and subscriber units in the DLCor NWK layers.

The D-field is the largest data field, and in the case of normaltelephony contains digitised speech samples, but can also containnon-speech data samples.

Provision is made in the preferred system for subscriber unitauthentication using a challenge response protocol. General encryptionis provided by combining the speech or data with a non-predicablesequence of cipher bits produced by a key stream generator which issynchronised to the transmitted super-frame number.

In addition, the transmitted signal is scrambled to remove dccomponents.

Multi-Bearer Operation

Data, such as digitised speech, is carried at a rate of 32 Kbps per slotof a TDMA frame. In cases where a faster rate is required, n-slots ineach frame are used, giving a rate of n×32 Kbps. Each assigned slot canbe considered to be a message bearer. Thus, when more than one slot perframe is assigned to a connection, the connection is designated amulti-bearer.

Multi-bearer operation is used to transmit voice-band non-speech datasuch as facsimile and modem data signals. These use 64 Kbps PCM coding.This data is sent via two slots per frame.

As shown in FIG. 3, a 64 Kbps data stream for transmission is dividedinto two 32 Kpbs data streams to be transmitted in different slots.Alternate octets (8 bits) are assigned to different slots, e.g. oddoctets to slot 0 and even octets to slot 1. The octets within a n×160bit sampling window are considered as successively numbered. Any pair ofslots besides slots 0 and 1 can, of course, be chosen to carry the data,including slot 0 on two different RF carriers. At the receiver, the oddand even received octets are interleaved to reconstitute the original 64Kbps message.

A multi-bearer connection is established, either by first establishing asingle bearer connection, and then increasing the number of bearers, orpaging the subscriber unit with a multi-bearer page request message. Thepaging procedure is similar to that for establishing a single bearerconnection; however, the paging message contains the bearer requirementsfor the entire connection and can assign up to six slots as bearers.

Creation of a Multi-Bearer Connection

During a connection it may be necessary to increase or decrease thenumber of bearers used. An example is where following the detection of avoice-band data modem call, a second bearer needs to be allocated. Thisis done by the base station, which adds the new bearers.

FIG. 4 illustrates the protocol for adding bearers to a connection. Inthat figure, and in FIG. 5, the following abbreviations are used:Cpte--complete; Ack--acknowledge; no--number; MS--MAC Signalling.

Data is transmitted over the preferred system in a layered protocolsimilar to that used in GSM. When required, a decision is made in theupper NETWORK layer to provide a 64 Kbps PCM service. An Add-Bearermessage is then sent via any established bearer indicating the bearer tobe added. There is then a training sequence on each new bearer, and oncecompleted, an Enable-Service message is sent by the base station to thesubscriber unit and this is acknowledged by the subscriber unit.Receiver muting is applied whilst the new bearers are being added. Thebase station then sends an Enable-Service-Cpte message to thesubscriber, which removes the muting. Once the subscriber ceases sendingthe acknowledgement signal to send another MS-Idle to the base station,the base station responds by removing the mute and the new multi-bearerservice is then fully enabled.

Reducing the Number of Bearers in a Multi-Bearer

To release a bearer which is no longer required, a decision is made inthe NETWORK layer of the layered protocol. The new bearers to be usedare identified, and bearers which have become redundant are released.FIG. 5 shows example control messages which are sent between thesubscriber unit and base station.

We claim:
 1. A method of transmitting a digital data message in timeslots within fixed length time frames between a sending unit and areceiving unit, a time slot or slots in successive frames being assignedfor transmission of the message, in which at least a portion of themessage is detected to determine a parameter of the messages, and saidparameter is used to deduce which, if any, of an assignment of at leastone further slot and a deassignment of at least one slot is appropriatefor transmission of the message, and in which the number of slots whichare used to carry the message is increased or decreased in accordancewith said deduction without terminating the message transmission betweenthe sending unit and the receiving unit, in which method selectable atleast one further slot is assigned dependent upon whether the messagecomprises speech, or non-speech voice-band data.
 2. A method oftransmitting a digital data message according to claim 1, by radio.
 3. Amethod of transmitting a digital data message according to claim 1, inwhich the message is allocated to the assigned slots for transmissionand reconstituted after reception at the receiving unit.
 4. A method oftransmitting a digital data message according to claim 1, in which thefirst slot or slots and the further slot or slots are included in eachframe.
 5. A method of transmitting a digital data message according toclaim 1, in which the sending unit is a base station and the receivingunit is a subscriber unit.
 6. A method of transmitting a digital datamessage according to claim 5, in which assignment of slots is undertakenby the base station.
 7. A method of transmitting a digital data messageaccording to claim 1, in which non-speech voice-band data is modem data,facsimile data or an ISDN message.
 8. A method of transmitting a digitaldata message according to claim 1, in which a second slot is assignedfor carrying a facsimile message such that two slots per frame are usedfor carrying the facsimile message unless a second slot is unavailabledue to the large amount of communication traffic at that time.
 9. Amethod of transmitting a digital data message according to claim 1, inwhich at least one further slot is assigned such that ISDN message arecarried on between two and five slots per frame.
 10. A method oftransmitting a digital data message according to claim 1, in which thesteps of detection to determine the parameter and further slotassignment are undertaken at the sending unit.
 11. A method oftransmitting a digital data message according to claim 1, in which thedetection step includes determining whether the message includes acharacteristic expected if the call includes voice-band data.
 12. Amethod of transmitting a digital data message according to claim 1, inwhich the characteristic is standard modem answertone or facsimilehandshake signalling or ISDN call set-up signalling.
 13. Communicationmeans for communicating a digital data message in time slots withinfixed length time frames, the communication means including a sendingunit and a receiving unit, at least one of said units including firstslot assignment means operative to assign a time slot or slots insuccessive frames, and at least one of said units including second slotassignment means operative to selectively assign at least one furtherslot or deassign at least one slot, the second slot assignment meansincluding detection means to detect at least a portion of the message todetermine a parameter of the message and including assignment means toselectively assign said at least one further slot or deassign at leastone slot dependent on the parameter without terminating the messagetransmission between the sending unit and the receiving unit, thesending unit including means operative to divide the message between theassigned slots, and the receiving unit including means operative toreconstitute the message, in which the assignment means selectableassigns at least one further slot dependent upon whether the messagecomprises speech, or non-speech voice-band data.
 14. Communication meansaccording to claim 13, in which the sending unit sends by radio. 15.Communication means according to claim 13, in which the receiving unitis at a fixed location.
 16. Communication means according to claim 13,in which the sending unit is a base station and the receiving unit is asubscriber unit.
 17. Communication means according to claim 16, in whichthe base station comprises the second slot assignment means.
 18. A basestation comprising transmission means for transmission of digital datasignals in time slots of fixed length time frames and reception meansfor receiving digital data signals in corresponding time slots of fixedlength time frames, the base station including means to assign a slot orslots in a frame for communication with a subscriber unit, means todetect at least a portion of the transmitted signal to determine aparameter of the transmitted signal, means to deduce from said parameterwhich, if any, of an assignment of at least one further slot and adeassignment of at least one slot is appropriate for communication withthe subscriber unit and means to increase or decrease the number ofslots used for communication with the subscriber unit in accordance withsaid deduction without terminating communication with the subscriberunit, in which the means to increase or decrease the number of slotsselectable assigns at least one further slot dependent upon whether themessage comprises speech, or non-speech voice-band data.
 19. A basestation according to claim 18, operative to transmit TDM digital datasignals and receive TDMA digital data signals.